Hydronic heating systems can provide comfort for your home, like this one in the floor.

Hydronic heating systems can provide comfort for your home, like this one on the wall.

The Heating Edge is a recent development in low-temperature fin-tube baseboard heating (detail).

The Heating Edge is a recent development in low-temperature fin-tube baseboard heating.

Tubing embedded in a concrete floor slab is the most common form of radiant floor heating.

A thin-slab radiant panel installation awaits the concrete pour. The 1/2-inch PEX-AL-PEX tubing has been carefully fastened using a special stapler. A layer of 6-mil polyethylene film provides a bond break between the slab and the plywood subfloor underneath.

A radiant ceiling system is installed in much the same way as a radiant wall system.

An infrared thermograph of a hydronic radiant ceiling as it is warming up. The water flow is from left to right, as shown by the red and orange areas.

JAGA North America’s Low H20 panels provide the latest in contemporary looks and thermal performance. Small fans move air past the radiator fins, increasing low-temperature output up to 250%.

Contemporary radiant panel designs, like this one from Vasco Heating Concepts, can look like a work of art in themselves. Be sure to verify low-temperature performance when choosing any radiant wall panel.

This radiator from Runtal is part baseboard, part wall panel. Radiant panels come in many shapes and sizes to fit almost any application.

This home run distribution system is simple and effective, with a manifold accessible through a wall panel.

A panel radiator with an integrated thermostatic radiator valve

A panel radiator with an integrated thermostatic radiator valve (detail).

Heated Thin-Slabs

Another common method of hydronic floor heating uses a 1.5-inch “thin slab” poured over a wooden deck. The slab can be either concrete or poured gypsum, but should never be lightweight concrete, which uses vermiculite or polystyrene beads instead of stone aggregate, and has significantly higher thermal resistance.

Because the slab is thinner, it has somewhat poorer heat dispersion characteristics, needing a slightly higher water temperature for a given rate of heat output—but this difference is slight: A 1.5-inch concrete thin-slab with 12-inch tube spacing and covered with a finish flooring resistance of 0.5°F/hr./ft.2/Btu yields about 8% less heat output than a 4-inch-thick slab with the same tube spacing and finish flooring. To get the same efficiency, 9-inch, rather than 12-inch, tube spacing can be used.

The following guidelines are suggested for thin slabs supplied by renewable heat sources:

Tube spacing for a thin-slab application should not exceed 9 inches.

Floors under thin-slabs should have minimum of R-19 underside insulation.

Floor finishes should have a total R-value of 1.5 or less (lower is always better).

Heated Walls & Ceilings

Walls and ceilings can also be turned into low-temperature hydronic radiant panels. These radiant walls are indistinguishable from a standard interior wall. Its low thermal mass lets it respond quickly to changing room load conditions or zone setback schedules. This fast response is especially important in homes with low heat loss or significant internal heat gain because such spaces can quickly overheat.

The panel’s rate of heat emission is approximately 0.8 Btu/hr./ft.2 for each 1°F the average water temperature in the tubing exceeds room air temperature. For example, if the average water temperature in the tubing is 110°F in a room with 70°F air temperature, each square foot of wall releases about 32 Btu per hour [0.8 × (110°F - 70°F)]. This average water temperature is well within the range of what most renewable energy heat sources can supply.

If you plan to install this system on the inside of an exterior wall, make sure the R-value of that wall is 50% higher than that of unheated exterior walls. That keeps the rate of heat loss to the outside about the same as for an unheated wall. If you’re installing this on an inside partition wall, use 3.5-inch fiberglass batt in the stud cavities behind the heated wall. Finally, radiant wall panels work best constructed no higher than 3 to 4 feet above floor level. These heights bias the radiant heat output into the occupied zone of rooms, and thus improve comfort.

Radiant ceilings use the same construction as a radiant wall. The only difference is that the materials are fastened to the ceiling framing rather than the studs. The infrared thermograph shows such a ceiling as it warms up. The red areas on the left side indicate that the aluminum heat transfer plates are dissipating heat away from the tubing and across the adjacent ceiling surfaces.

Like the radiant wall, a radiant ceiling has low thermal mass and can respond quickly to interior temperature changes. Heated ceilings also have the advantage of not being covered by rugs or furniture, and thus are likely to retain good performance over the building’s life, but can be a bit more expensive relative to a heated slab-on-grade floor.

Comments (9)

With any renewable type system the rule should be make as much as you can whenever you can then use what you gain wisely, eg. do laundry when the sun is shining. The simple solution for moderating high temperatures especially with water systems is a mixing valve to only use what you need and leave the rest for other uses.Same with electricity.

I was looking at a small room heating idea using that under floor electric heat Mat stuff instead of a liquid based solution. How do these compare to one another in terms of cost, installation and efficiency?

The space is a small 12X13 bedroom I plan on building this summer in my walk out basement. The location is on the north side where it's going to get chilly.

I think Suzan's latest comments are spot on, but it depends on the specific situation and climate. It's almost always a balance between cost and benefit, resources and results.

In many climates (like mine), there isn't a lot of sun when space heat is needed, so focusing on solar space heating doesn't make a lot of sense. And when it does, passive solar design is usually a simpler and more cost effective option.

In general (again, specific homeowner goals and the specifics of the site, climate, and house will affect this greatly), I would focus first and foremost on efficiency, thermal and otherwise. Then I would look at efficiency of the heating system, with mini-split heat pumps being the current star performer in the cost/benefit arena. Then I would go after domestic water heating, using a solar hot water system. And then PV.

That said, recent drops in the cost of PV, and its simplicity compared to SHW, lead many people to go for PV sooner in their priority list. And each person has different goals, budget, patience for complexity, and attention span for RE and efficiency work. Many of my students and clients choose only to invest in PV, 'cause it's easy and effective, even after I've advised efficiency work first.

I think all work towards RE and efficiency is not a waste of time, and we each have different goals and situations. Active solar thermal _space heating_ is often of questionable cost effectiveness in my experience, in my moderate, cloudy-winter climate.

"Then I would look at efficiency of the heating system, with mini-split heat pumps being the current star performer in the cost/benefit arena."

Do you have some numbers to show this so that an apples to apples comparison can be done?? Can you point me at some spec sheets for some heat pumps and I can do my own analysis? (I'm an engineer)

I agree with the thermal efficiency statement, we still build lousy houses but the house you have is the house you have. A lot of articles (like this one) don't seem to take into account the retrofit market. I'm looking at the heating for my own home (presently baseboard electric) as my last electric bill was $600 (of course most of the charges were not for the actual electricity but that's another story).

"That said, recent drops in the cost of PV, and its simplicity compared to SHW, lead many people to go for PV sooner in their priority list."

In my area (Ontario, Canada), I would say this is because our government has made a large investment in solar programs (paying $0.83/kWH for solar when rate from utility is $0.08-0.10) and people jump on the bandwagon. From and engineering efficiency point of view solar still doesn't make a lot of sense (IMO). By the time the system is paid off (20 yrs), it's time for a new one (20 yr expected lifetime).

I don't have cost comparisons handy for heating systems, and I'm out of the country with poor connectivity at the moment too. But I encourage you to do more research on mini-split air-source heat pumps. The COPs claimed are in the 2-5+ range, which means 2 to 5+ times the heat for your kWh. And the cost is very modest -- in the $4-10K range installed complete, depending on home size and number of indoor units.

When I referred to the recent drops in PV, I was not talking about incentives at all, but the actual installed costs of systems, which has come down dramatically in the last several years, due to lower prices on PVs primarily. PV _very_ often makes purely financial sense with incentives, and it makes even more environmental sense. PV modules have _warranties_ of 20-25 years, and will be producing for 40+ years if well installed and maintained. I have modules on my roof that were installed in 1984 and are still going strong. Even if your very low prediction (20 years) were true (it is not), what else can you buy that is _productive_ and lasts that long? PVs are an amazing product that is very underrated.

I orginally got interested in solar because I live in Maine where the vast majority of people have a boiler fired by oil. It seems intellegent to use solar hot water. What I learned was regular baseboard was designed for 180 degree which is not realistic from solar in the winter. So I went about recommending everyone build a new home with radiant floors. I have now changed my mind. There is no logical reason to build an inefficient home. Proper air sealing and insulation is not very expensive. If you build right a simple air source heat pump, our favorite is Mistubishi mini split, is enough to heat the home. For really long term cold you may need a simple electric space heater, but those can be had for $30. Since energy efficient homes dont loose heat fast a day of cold temps will have little impact on the indoor temp even if the heat pump cannot produce. These days heat pumps run down to -17 degrees, that covers 99.9% of the days in Maine. Heat pumps are MUCH less expensive than solar powered radiant heat or low temp baseboard & radiators. The savings can go into air sealing and insulation. A small solar hot water system can meet domestic needs.

Suzan, You have come to the same conclusion that I often do -- thermal energy efficiency coupled with mini-split air-source heat pumps is the best option. In addition to the excellent reasons you cite, shifting the heating load to electricity also means that you may be able to power your heating system renewably, either through an on-site PV/wind/hydro system, through the renewable electricity your utility already sells, or through renewable energy credits.

Is there any benefit to using a water-to-water "geothermal" heat extraction device to remove heat from the fluid entering the solar thermal circuit? Would this, in your opinion, contribute to efficiency if the heat obtained via "geothermal" was used to augment heat gains obtained through the solar thermal loop?